In the past, there have been many devices for causing a jet of ink to flow onto a paper target. For example, U.S. Pat. No. 3,060,429 issued to Teletype Corporation on Oct. 23, 1962. In the Teletype patent, a positive pressure is applied to the ink upstream of the nozzle. The nozzle is a simple round orifice or aperture. An electric field is impressed between the nozzle and the paper target to cause the ink to flow to the paper. Various plates or valves are disposed between the nozzle and the target and operate in response to control means to turn the ink flow on or off.
An improvement on the Teletype style nozzle in U.S. Pat. No. 3,886,565 issued on May 27, 1975 to Tokyo Shibaura Electric Co., Ltd. The Tokyo Shibaura patent shows an apertured nozzle which has a sharp extended point formed on a downstream lip portion. The ink is acted upon by a positive upstream pressure.
The well-known conventional IBM 6640 ink jet printer system is described in the April, 1979 issue of Scientific American, p. 162 ("Ink Jet Printing"). This system uses an apertured nozzle and directs an ink stream composed of relatively large droplets towards the target. A gutter between the nozzle and the target is used to intercept approximately 98% of the ink droplets before they hit the target. The intercepted ink is recirculated to the ink supply. These prior art devices (and other prior art devices known to applicant) do not achieve the following goals that applicant's invention achieves:
(a) to deposit a sharp image on the target by substantially suppressing the ink jet stream from turning into relatively large ink droplets.
(b) to deposit an image on the target which has a gray scale by directly modulating the amplitude (i.e. flow rate) of the ink jet.
(c) to permit a large number of nozzles to be used in close proximity to each other and to the target by eliminating all implements or structures between the nozzles and the target, and by preventing electrical interference between nozzles.